A charge/discharge condition which is adoptable in a sodium-sulfur battery depends on a state of charge of a sodium-sulfur battery. For example, as a state of charge of a sodium-sulfur battery gets closer to the last stage of charge, power which can be discharged by a sodium-sulfur battery is increased, and a time during which a sodium-sulfur battery can continue discharging is made longer. Then, power which can be charged into a sodium-sulfur battery is reduced, and a time during which a sodium-sulfur battery can continue being charged is made shorter.
A charge/discharge condition which is adoptable in a sodium-sulfur battery depends also on a temperature of a sodium-sulfur battery. A reason why a charge/discharge condition adoptable in a sodium-sulfur battery depends on a temperature of a sodium-sulfur battery is that while heat generation or heat absorption occurs in a sodium-sulfur battery when a sodium-sulfur battery discharges and is charged, an upper limit of temperature which should not be violated, such as a maximum allowable temperature and a recommended operation temperature, is determined for a sodium-sulfur battery. A reason why heat generation or heat absorption occurs in a sodium-sulfur battery when a sodium-sulfur battery discharges and is charged is that a cell reaction which is an exothermic reaction proceeds in a sodium-sulfur battery when a sodium-sulfur battery discharges, a cell reaction which is an endothermic reaction proceeds in a sodium-sulfur battery when a sodium-sulfur battery is charged, and Joule heat is generated due to internal resistance in a sodium-sulfur battery when a sodium-sulfur battery discharges and is charged.
A charge/discharge condition which is adoptable in a sodium-sulfur battery depends also on a deterioration state of a sodium-sulfur battery. A reason why a charge/discharge condition which is adoptable in a sodium-sulfur battery depends also on a deterioration state of a sodium-sulfur battery is that internal resistance which causes Joule heat as described above varies with a deterioration state of a sodium-sulfur battery.
In some cases, a charge/discharge condition which is adoptable in a sodium-sulfur battery depends on the other factors than a state of charge, a temperature, and a deterioration state of a sodium-sulfur battery.
As described above, a charge/discharge condition which is adoptable in a sodium-sulfur battery depends on many factors such as a state of charge, a temperature, and a deterioration state. Thus, numerous calculations are needed in order to calculate a charge/discharge condition which is adoptable in a sodium-sulfur battery. Also a circumstance where charge power should be reduced as the last stage of charge approaches in the neighborhood of the last stage of charge when a sodium-sulfur battery is charged, or the like, is a cause of a need for numerous calculations. Charge power should be reduced as the last stage of charge approaches when a sodium-sulfur battery is charged, because internal resistance of a sodium-sulfur battery is sharply increased in the neighborhood of the last stage of charge so that it is necessary to reduce a voltage applied to a sodium-sulfur battery by reducing a current flowing in a sodium-sulfur battery in the neighborhood of the last stage of charge.
Meanwhile, it is desired to provide an operator of an electrical energy storage device including a sodium-sulfur battery with information rendering assistance in driving the electrical energy storage device. A technique described in Japanese Patent Application Laid-Open No. 2008-210586 is one example thereof.